Hamid Almasi

Enhanced tunneling magnetoresistance and perpendicular magnetic anisotropy in Mo/CoFeB/MgO magnetic tunnel junctions

Structural, magnetic, and transport studies have been performed on perpendicular magnetic tunnel junctions (pMTJ) with Mo as the buffer and capping layers. After annealing samples at 300 °C and higher, consistently better performance was obtained compared to that of conventional pMTJs with Ta layers. Large tunneling magnetoresistance (TMR) and perpendicular magnetic anisotropy (PMA) values were retained in a wide range of samples with Mo layers after annealing for 2 h at 400 °C, in sharp contrast to the junctions with Ta layers, in which superparamagnetic behavior with nearly vanishing magnetoresistance was observed. As a result of the greatly improved thermal stability, TMR as high as 162% was obtained in junctions containing Mo layers. These results highlight the importance of the heavy-metal layers adjacent to CoFeB electrodes for achieving larger TMR, stronger PMA, and higher thermal stability in pMTJs.

Anomalous spin-orbit torque switching in synthetic antiferromagnets

We report that synthetic antiferromagnets (SAFs) can be efficiently switched by spin-orbit torques (SOTs) and the switching scheme does not obey the usual SOT switching rule. We show that both the positive and negative spin Hall angle (SHA)-like switching can be observed in Pt/SAF structures with only positive SHA, depending on the strength of applied in-plane fields. A new switching mechanism directly arising from the asymmetric domain expansion is proposed to explain the anomalous switching behaviors. Contrary to the macrospin-based switching model that the SOT switching direction is determined by the sign of SHA, the new switching mechanism suggests that the SOT switching direction is dominated by the field-modulated domain wall motion and can be reversed even with the same sign of SHA. The new switching mechanism is further confirmed by the domain wall motion measurements. The anomalous switching behaviors provide important insights for understanding SOT switching mechanisms and also offer novel features for applications.

Effect of Mo insertion layers on the magnetoresistance and perpendicular magnetic anisotropy in Ta/CoFeB/MgO junctions

C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program - MARCO; DARPA; National Science Foundation [ECCS-1310338]

Perpendicular magnetic tunnel junction with W seed and capping layers

C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program; MARCO; DARPA; National Science Foundation [ECCS-1310338]; Inston, Inc., through a Phase II Small Business Innovation Research award from the National Science Foundation

Voltage-controlled interlayer coupling in perpendicularly magnetized magnetic tunnel junctions

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

Superparamagnetic perpendicular magnetic tunnel junctions for true random number generators

Superparamagnetic perpendicular magnetic tunnel junctions are fabricated and analyzed for use in random number generators. Time-resolved resistance measurements are used as streams of bits in statistical tests for randomness. Voltage control of the thermal stability enables tuning the average speed of random bit generation up to 70 kHz in a 60 nm diameter device. In its most efficient operating mode, the device generates random bits at an energy cost of 600 fJ/bit. A narrow range of magnetic field tunes the probability of a given state from 0 to 1, offering a means of probabilistic computing.

Magnetic reversal and thermal stability of CoFeB perpendicular magnetic tunnel junction arrays patterned by block copolymer lithography

Dense arrays of pillars, with diameters of 64 and 25 nm, were made from a perpendicular CoFeB magnetic tunnel junction thin film stack using block copolymer lithography. While the soft layer and hard layer in the 64 nm pillars reverse at different fields, the reversal of the two layers in the 25 nm pillars could not be distinguished, attributed to the strong interlayer magnetostatic coupling. First-order reversal curves were used to identify the steps that occur during switching, and the thermal stability and effective switching volume were determined from scan rate dependent hysteresis measurements.

Saturation of VCMA in out-of-plane magnetized CoFeB/MgO/CoFeB magnetic tunnel junctions

Voltage controlled magnetic anisotropy (VCMA) currently attracts considerable attention as a novel method to control and manipulate magnetic moments in high-speed and low-power spintronic applications based on magnetic tunnel junctions (MTJs). In our experiments, we use ferromagnetic resonance (FMR) to study and quantify VCMA in out-of-plane magnetized CoFeB/MgO/CoFeB MTJ pillars. FMR is excited by applying a microwave current and detected via a small rectified voltage which develops across MTJ at resonance. The VCMA effective field can be extracted from the measured resonance field and was found to vary as a function of electrical bias applied to MTJ. At low applied biases, we observe a linear shift of the VCMA field as a function of the applied voltage which is consistent with the VCMA picture based on the bias-induced electron migration across the MgO/CoFeB interface. At higher biases, both positive and negative, we observe a deviation from the linear behavior which may indicate a saturation of the VCM...

Probing VCMA in MTJs with in-plane magnetization

Voltage controlled magnetic anisotropy (VCMA) is a novel method to switch magnetizations in low-power and ultra-fast applications based on magnetic tunnel junctions (MTJs). Here we explore the ferromagnetic resonance (FMR) technique to probe VCMA in situations where other methods cannot be applied. We quantify VCMA in CoFeB/MgO/CoFeB MTJ nanopillars with in-plane magnetizations where our FMR method is unique in providing direct information about VCMA. We observe a quadratic shift of the FMR resonance field when a voltage bias is applied across the MTJ. The VCMA energy corresponding to the quadratic shift varies with an energy factor of 8.2μJ/m2 for 1 V2/nm2. These results are important for understanding magnetodynamics in MTJ-based applications with in-plane magnetizations.

Investigation of Layer Composition and Morphology in Perpendicular Magnetic Tunnel Junctions

As traditional complementary metal–oxide semiconductor (CMOS) technology approaches its limit, alternative technologies such as magnetic tunnel junctions (MTJs) are being explored to replace CMOSbased devices for memory and logic applications. MTJs have advantages such as nonvolatility, low power consumption, and high densities [1]. These features have enabled application in technologies such as magnetic random access memory (MRAM), static random access memory (SRAM), and spin-transfer torque MTJs (STT-MTJs).